Worried...HELP!!

Ok like i said im 13 years old 5'3 and now 109 pounds i lost one pound from yesterday from not eating all day and well i have been having some bad pains in my back and stomach it hurts like all the time now and i dont know wut to do i cant tell my mom cuz she already thinks im anerexic and well i just dont like to eat everyday cuz i dont want to get fat and im to scared to tell my bestfriend jessica because she might go to my mom and well i think it will all get better but it keeps getting worse and worse

You really need to talk to someone about this. Fair enough if you don't want to tell your mum the details, but you must talk.
Whatever you think, if you are going whole days without eating and say things like "I don't like to eat every day in case I get fat" then you at least have the beginnings of an eating disorder. Clearly, if your mum apparently thinks you're anorexic then she has been picking up on something. And unfortunately she might just be right.
Good luck.